Grating coupled vertical cavity optoelectronic devices

Abstract

A edge emitting waveguide laser is obtained that derives its optical power from a vertical cavity laser structure. The vertical cavity laser with top and bottom Distributed Bragg Reflectors produces stimulated emission by resonance in the vertical direction but the optical power so generated is diffracted by a second order grating into an optical mode propagating in the optical waveguide formed by the upper and lower mirrors as cladding layers. The efficiency of the diffraction grating and the reflectivity of the mirrors are maximized so that essentially all of the light is coupled into the guide and the loss through the mirrors can be neglected. The same structure can be utilized as a detector, a modulator or an amplifier. The designated laser structure to achieve this form of operation is the inversion channel laser which is a laterally injected laser having both contacts on the top side of the device. Then the anode and cathode of the laser are essentially coplanar electrodes and the device is implemented in the form of a traveling wave laser, detector, modulator or amplifier which forms the basis for very high frequency performance.

Description

TECHNICAL FIELD OF THE INVENTIONThis invention relates to the field of semiconductor double heterostructure laser devices and, in particular, to those laser devices which use vertical cavities. It also relates to the field of corrugated optical waveguides and travelling wave optoelectronic devices.BACKGROUND OF INVENTIONNext generation transmission systems are anticipating bit rates approaching 100 Gb / s in time division multiplexed architectures. The demand for such speeds is created by the growth of interactive multi-media services and is made possible by the terahertz bandwidth of optical fiber. Realizing optical sources with those modulation bandwidths remains a significant obstacle however. The state-of-the-art edge-emitting semiconductor lasers have 3 db bandwidths in the region of 30 GHz. The limitations on this bandwidth arise from the non-linear gain mechanism and from the maximum values of differential gain that can be realized. The non-linear gain effect is due to the pres...

AI Technical Summary

Benefits of technology

The device is described in one illustrative embodiment of the HFET inversion channel laser, which comprises one of the laser devices in the general family of inversion channel optoelectronic devices which are modulation doped devices. In this form of the invention, a refractory metal emitter provides a conduction path of hole flow into the laser active region by two-dimensional conduction. The two dimensional contour of the conduction with path is established by an N type implant under the metal emitter. The refractory metal emitter is constituted of two metal stripes one positioned on either side of an optical waveguide which utilizes the upper DBR mirror of the vertical cavity to provide its cladding. Separation of the refractory metal contact into two stripes enables a waveguide to transport the light diffracted into the channel from the vertical cavity laser mechanism by the action of the grating without losses from optical scattering at the metal. The device is a laterally injected laser and thus ion-implanted source contacts provide electron flow to the active laser channel, i.e. the inversion channel. The source contact metals stripes and the emitter metal strips form the electrodes for a coplanar transmission line. The device is designed for equal electrical and optical velocities.

Problems solved by technology

Realizing optical sources with those modulation bandwidths remains a significant obstacle however.

The limitations on this bandwidth arise from the non-linear gain mechanism and from the maximum values of differential gain that can be realized.

However, in the edge emitter these regions cannot be reduced to much less than about 1500 Å in thickness and yet still maintain a reasonable value for I′, the optical confinement factor for the quantum well in the optical waveguide.

Also, as the low index waveguide regions are placed closer to the quantum well, the large index difference interfaces produce larger waveguide loss.

This limit is imposed by the rapidly rising threshold current and the reduced power capability.

The larger threshold, of course, results in lower maximum power due to the reduced current range for optical output and the increase in internal device heating.

The limiting factor in these advanced structures has been the RC time constant of the device.

In the edge-emitting laser, this bandwidth limitation results from the device parasitic capacitance (bond pad plus intrinsic PIN capacitance) and the output resistance of the measurement system because the device series resistance can be made very small.

In the vertical cavity device, the device series resistance cannot be made negligibly small because the conduction is either forced through part of a DBR mirror or suffers from current crowding effects.

Another impediment to vertical cavity deployment is the coupling of the light to fibers.

This is not a cost effective approach.

At speeds of 100 GHz, hybrid connections of lasers and transistors become costly and impractical.

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